1. Field of the Invention
The present invention relates to a fuel supply control method for internal combustion engines and, more particularly, the present invention relates to a fuel supply control method for an internal combustion engine having fuel injection valves provided upstream and downstream of the throttle valve in the suction pipe, respectively, to supply fuel to a plurality of cylinders.
2. Description of the Prior Art
A conventional fuel supply control system as disclosed in Japanese Provisional Patent Publication (Kokai) No. 47-35422 for distributing fuel injected by common fuel injection valves to the cylinders of an internal combustion engine controls the fuel supply so that fuel is supplied by a main fuel injection valve provided upstream of the throttle valve while the engine is operating in a middle load or high load mode, and fuel is supplied by an auxiliary fuel injection valve provided downstream of the throttle valve while the engine is operating in a low load mode. This fuel supply control system employs a fuel injection valve having excellent atomizing characteristics as the auxiliary fuel injection valve to secure uniform fuel distribution to all the cylinders during the low load operation of the engine, in which fuel is injected at a low rate.
According to this known fuel supply control method, fuel is allowed to flow smoothly without being obstructed by the throttle valve even when the throttle valve is fully or almost fully closed during the low load operation of the engine and hence fuel can be supplied to the cylinders with high responsiveness to the operating condition of the engine.
Fuel is satisfactorily atomized in the suction pipe when the engine is warm. Therefore, it is desirable to inject fuel by the auxiliary fuel injection valve having a fuel injection rate lower than that of the fuel injection valve disposed upstream of the throttle valve to supply fuel to the cylinders with high responsiveness to the engine operating condition and to secure accurate control of fuel being supplied at a very low fuel injection rate.
However, when the engine is cold, fuel cannot satisfactorily be atomized in the suction pipe because fuel is supplied at an increased rate by increasing the fuel injecting period of the auxiliary fuel injection valve and the suction pipe is cold. If the auxiliary fuel injection valve is disposed downstream of the throttle valve and near the intake manifold to avoid deterioration of fuel atomization, fuel injected by the auxiliary fuel injection valve cannot uniformly be distributed to the cylinders.
Further, in the conventional fuel supply control system, the fuel amount actually supplied to the cylinders varies if the working fuel injection valve is changed over from the auxiliary fuel injection valve disposed downstream of the throttle valve to the fuel injection valve disposed upstream of the throttle valve in response to change of the operating mode of the engine from the low load mode to the high load mode while the engine is cold. This is by the following reason: Upon the change of the operating mode the fuel injecting operation of the auxiliary fuel injection valve disposed downstream of the throttle valve injecting nearly the maximum fuel quantity is interrupted instantly and the fuel injection valve disposed upstream of the throttle valve starts injecting fuel at a high fuel injection rate. On such an occasion, dry inner surfaces of the throttle body and the suction pipe in the vicinity of the throttle valve are wetted by fuel injected by the upstream fuel injection valve. Since the lower the temperature of the inner wall of the intake pipe, the greater the amount of fuel that wets the inner surfaces of the throttle body and the suction pipe, the actual fuel amount supplied to the cylinders varies greatly at the time of changeover of the working fuel injection valve when the engine is cold. Accordingly, the aforementioned fuel supply control amount in changing over the working fuel injection valve from the auxiliary fuel injection valve to the upstream fuel injection valve.
In an internal combustion engine having a fuel supply system including a fuel injection valve for supplying fuel in common to a plurality of cylinders, disposed upstream of the throttle valve disposed upstream of the intake manifold, such as one stated above, a fuel pressure regulator regulates the fuel pressure, namely, the pressure of fuel supplied to the fuel injection valve, at a pressure higher by a fixed amount than pressure prevailing within the suction pipe in the vicinity of the nozzle of the fuel injection valve projected thereinto, namely, pressure within the suction pipe at a location upstream of the throttle valve to maintain the discharge pressure of the fuel injection valve constant. Such a fuel supply system is able to accurately meter the fuel supply amount according to the fuel injection period of the fuel injection valve. However, the same is unable to accurately meter the fuel supply amount over a wide range of fuel supply rate.
To deal with such a problem, Japanese Patent Publication (Kokoku) No. 61-2775 has proposed a fuel supply control method, which controls the pressure regulator to regulate the pressure of fuel supplied to the fuel injection valve disposed upstream of the throttle valve according to the pressure within the suction pipe at a location downstream of the throttle valve to secure accurate fuel supply amount over a wide range of fuel supply rate.
This proposed fuel supply control method also has a disadvantage. That is, the negative pressure within the suction pipe downstream of the throttle valve varies during operation of the engine in a low load mode such as an idling mode because the flow rate of intake air is controlled to vary in order to stabilize the engine speed at an idling speed according to variation of the engine speed, and variation of the magnitude of external load such as the air conditioner and the power steering system, and this variation of the intake air flow rate in turn causes variation of the suction pipe pressure downstream of the throttle valve, which makes it difficult to carry out accurate control of fuel supply so as to meet fuel demand. Consequently, highly accurate control of the air-fuel ratio of the mixture, which is particularly important while the engine is operating in a low load mode such as an idling mode, is difficult to carry out, rendering the combustion of fuel unstable and hence deteriorating exhaust emission characteristics of the engine.
On the other hand, while the engine is operating in an operating mode other than a low load mode, i.e. a middle load or high load mode, the fuel supply amount from the fuel injection valve need not be controlled very accurately. In a middle load or high load the fuel supply amount is rather to be controlled properly over a wide range of fuel supply rate to improve the driveability of the engine.